The present invention relates to a power steering system for automobiles and the like, and in particular, to a power steering system which provides excellent steering feeling.
In automobiles, users thereof belonging to various classes, the power steering systems are widely equipped from large cars to compact cars, and are serving for reducing the fatigue and for ensuring the safety in driving.
As the power steering systems, heretofore, the hydraulic pressure type has been used primarily. However, in recent years, in view of the utility in providing variety of controls and in wasting less energy, electro-motive type power steering systems have been proposed.
In the electro-motive type power steering systems, such type of systems have primarily been employed in which in order to obtain a large operating force required for an electro-motive type actuator used in the power steering systems, an electric motor is used as this actuator, and its output is retarded to obtain a final operating force for assisting the steering.
However, in the conventional electro-motive type power steering system, the drawback is involved in that since the assisting steering force is provided through a reduction mechanism from the electric motor rotating at a high speed, a characteristic of the power steering system which is very undesirable for the steering feeling appears in the handling of the steering wheel due to an inertia of the electric motor, and due to a large frictional resistance encountered when the electric motor is rotated without energization thereof by exerting a force from the output side of the reduction mechanism.
As to the characteristic which appears in the handling of the steering wheel due to the inertia of the motor, and which is undesirable for the steering feeling, a method of utilizing a differential characteristic in the control of the motor is considered to be useful as proposed, for example, in Japanese Patent Unexamined Publication No. 55-76760 (1980), and this method is expected to provide considerable effects in improving the steering feeling.
However, this method requires a differential circuit and the like, and the provision of the differential characteristic, in turn requires a new countermeasure for noise, and thus an increase in manufacturing cost results. Against the degradation of the steering feeling due to the frictional resistance, no particular improving effect is expected, and as a whole, a satisfactory improvement in the steering feeling can not be obtained. In this respect, this degradation of the steering feeling due to the frictional resistance appears as a significant decrease in a restoring force which is normally present when the steering angle is other than zero (a neutral position), or appears as an extinguishment of the restoring force.
Furthermore, when a steering force is applied to the steering wheel, it is designed that an assisting force is added by the motor. However, since the motor has the inertia, in order to accelerate the motor, the driver has to promote or pull the motor by the steering force exerted by the steering wheel so that inertia energy of the motor proportional to the square of a rotational speed of the motor is added, and also in order to stop the motor, the driver has to assist to stop the motor through the steering wheel. In particular, when the driver tries to turn the steering wheel fast, he will feel a shock at the driver's hands due to a load developed by a delay in time until the motor reaches a required rotational speed. In order to compensate for this load, there is a control circuit which employs the differential characteristic as mentioned earlier. However, this method involves a drawback that since the energy for acceleration and deceleration needed for the motor is not taken into account, the energy which is compensated for by the steering speed is insufficient.
On the other hand, in recent hydraulic type power steering systems, it is proposed to improve the straight advancing performance by providing a restoring force in the power steering system as disclosed in Japanese Patent Unexamined Publication No. 54-90726 (1979). However, there has been entirely no consideration as to how to control the restoring force at the time of forward acceleration, backing, and normal travelling to enable to improve the safety during running and the straight advancing performance of the automobile.